Minnesota Vikings running back Adrian Peterson (28) has returned to full form this NFL season after this devstating knee injury last Dec. 24. / Haraz N. Ghanbari, AP

by Gary Mihoces, USA TODAY Sports

by Gary Mihoces, USA TODAY Sports

But the way Adrian Peterson is galloping through the NFL this season is a tribute to his grit -- and the surgical and rehabilitation advances that give the Minnesota Vikings running back a leg up on players from decades past.

Last Christmas Eve in Washington, Peterson took a hit from the side by Redskins safety DeJon Gomes that bent his left knee gruesomely inward. Peterson tore the anterior cruciate ligament (ACL), which connects the shin and thigh bones within the knee, and the medial collateral ligament (MCL) and damaged cartilage.

After his intensive "work and grind" to get back, Peterson enters Sunday against the visiting Chicago Bears having started every game this season. He leads the NFL with 1,446 rushing yards, 308 more than No. 2 Marshawn Lynch of the Seattle Seahawks. Even by Peterson's own lofty standards, that's his second highest total in six NFL seasons.

"The ACL injury, I knew it would be the toughest thing I had," Peterson, 27, said Thursday at the Vikings complex in Eden Prairie, Minn. "I had to work extremely hard to get back. I had to put in a lot of hours working hard, hard, non-stop to get back."

Peterson, who didn't play in preseason, is coming off 210 rushing yards against the Green Bay Packers, including an 82-yard touchdown. He's not surprised by his success.

"You've got to have one believer. I'm sure there weren't many out there, but I definitely believed I could come back and be better than I was before," Peterson said.

Yards don't measure his journey since surgery last Dec. 30, when his torn ACL was replaced by a graft from the patellar tendon of his kneecap, anchored on either end by screws. The surgery was performed by James Andrews, orthopedic surgeon to the stars of sports, including New Orleans Saints quarterback Drew Brees, former Dallas Cowboys quarterback Troy Aikman and former baseball ace Rogers Clemens.

"Hearing about ACLs ... some guys take longer to bounce back from it. I knew that," Peterson said. "I just didn't know how intense it would be until I started the process."

Multiple medical articles have repeated the line from the early 1970s that the torn ACL was "the most common cause of the ex-athlete." That has changed in the past three decades with the advent of reconstructions using grafts. The procedure's emergence in the late 1980s benefited from the advent of arthroscopic surgery, which allows surgeons to see inside the knee as they work via fiberoptic cameras.

Kansas City Chiefs running back Jamaal Charles, also coming off ACL surgery, has another 1,000-yard season going and ranks sixth in the NFL in rushing. Players rehabbing from the surgery this season include New York Jets cornerback Darrelle Revis and Houston Texans linebacker Brian Cushing.

Peterson has become Exhibit A of its effectiveness and an inspiration for those in rehab.

"You look at a guy like Adrian Peterson ... where the exact same thing happened to him last year and he had the exact same surgery. And you say like, 'You know what? I can come back from this,' " said Philadelphia Eagles center Jason Kelce, who tore his ACL and MCL in a Sept. 16 game against the Baltimore Ravens.

NFL players in 'high-risk zone' for ACL injury

ACL tears typically are caused by a blow to the knee from the outside or a sudden stop.

"I was blocking a defender and my right leg was planted. A safety went to tackle the running back ... and (he) just actually ended up diving straight right into my knee, from the outside in," Kelce said.

The injury is common in football. This season, the University of Maryland lost four quarterbacks with ACL tears.

Yet studies show women athletes are more likely to sustain ACL injuries. The American Academy of Orthopedic Surgeons lists varying estimates of the risk for women at two to 10 times higher than for men.

"I think it has to do with different dimensions of their height and weight and body mass index, and their muscles may not be as strong," said Christopher Harner, an orthopedic surgeon and medical director of the Center for Sports Medicine at the University of Pittsburgh Medical Center (UPMC).

NFL players, however, are in a uniquely high risk zone because of the stopping and starting, the cutting and unexpected blows to the knee. But the injury no longer means a player's career is over. Doctors generally say an athlete can return in 6-9 months after surgery.

"We've had seven players on our football team that have had this surgery before and are playing now," Jets Coach Rex Ryan said after Revis' injury. " â?¦ I don't think there's any doubt he's going to come back strong from it."

The repairs come with no guarantees. Doctors at the Richmond Bone and Joint Clinic in Sugarland, Texas, found in 2010 that 31 of 49 (63%) of NFL players studied returned to play in the league an average of 10.8 months after surgery.

There is a chance of re-injury. Nose tackle Casey Hampton of the Pittsburgh Steelers has had three ACL reconstructions: one on his left knee while at the University of Texas and two on his right knee with the Steelers, including one this past offseason.

But NFL players have a much better shot at coming back from knee injuries than they did decades ago, when careers were shortened or ended -- most notably that of Hall of Fame running back Gale Sayers, who played for the Chicago Bears from 1965-71.

Harner, an orthopedic surgeron for 25 years, said there have been pivotal advancements in the surgery.

"Putting (the graft) in the right spot, using the athlete's own tissue and returning them cautiously with correct rehab. Those are the three keys," said Harner, president of the American Orthopedic Society for Sports Medicine.

Over the years, surgeons have tried synthetic replacements; Harner said they haven't worked. Grafts from deceased donors still are used. But Harner said the optimal graft is from the patient's own body, such as the patellar tendon or a hamstring tendon.

"I think your own tissue is better," he said.

Freddie Fu, also an orthopedic surgeon at UPMC, echoes that.

"Studies have shown that there is a slightly increased risk of re-rupture in the young, active athletic population with (grafts from cadavers)," said Fu, chair of the Department of Orthopedic Surgery at the University of Pittsburgh School of Medicine.

With the options in place now, is a reconstructed ACL as good as the original?

Fu said it is "not the same" but that surgeons are working to make the replacement as close to the original as possible: "We feel that we are re-creating a knee that is very similar to the pre-injury knee."

Prehab and rehab

Surgeons typically don't reconstruct ACLs until a week or two after the injury. That interim is for the prehab.

"The best thing to do before surgery is to get as much swelling out of the joint as possible to get their range of motion back," said Anna Hartman, a physical therapist and Director of Physical Performance Therapy at Phoenix-based Athletes' Performance, a training/rehab center for elite athletes.

"The more range of motion they go into surgery with and the less swelling â?¦ the better the outcomes they have."

That means ice and elevation, massage and manipulation of the knee and exercising muscles of the hip that shut down after a leg injury. Hartman also has her patients work on range of motion in a pool with his or her body unweighted.

After surgery, Hartman said the doctors typically have the athletes stay home for 2-3 days. The knee is in an immobilizer (a type of brace that can be adjusted to allow for varying degrees of knee bending), and the athlete is on crutches.

After that, the rehab moves into full swing. Hartman said athletes usually are off crutches in two weeks, although the immobilizer will stay on for about four weeks.

From five weeks to 16 weeks, she said, the athlete does exercises to strengthen the muscles of the hip, thigh and calves. "Typically around 12 weeks, we'll clear them to start running straight ahead," she said.

The cutting and running come between 4-6 months.

"Six months, usually, the doctor will clear them to return to sports-specific activities," Hartman said.

She added, "I won't allow somebody to cut until they've shown me they have good strength and hip stability and are tolerating things well."

The emphasis is on the quality, not the quickness, of the recovery.

"In the early 2000s, there were a lot of people pushing -- without any science -- returning in like 2-4 months," Harner said. "Many of these athletes weren't ready. They didn't have the muscle power. They didn't have the balance. ... Their knee would fatigue, and they'd blow the graft out again.

Now, the rehab is more carefully monitored, Harner said: "We're now looking at what is the function of the knee and the leg and the lower extremity. Can they do a shuttle drill (changes of direction around cones)? Can they do one-legged hops and can they run on a treadmill for 15 minutes and then stand on a single leg without having it wobble?"

Peterson rehabbed at the Vikings complex and near his Houston home, at Memorial Hermann Sports Medicine Institute, where he worked with physical therapist Russ Paine.

"The things you guys don't see is how much I work and grind and fought through different situations to get back," Peterson said. "Mentally, I was able to push through when I was tired and didn't want to do anything. I definitely give credit to the things I put into my off-season."

The patella tendon is still sore after games, but it's a small price to pay for Peterson to return to the form that makes a 2,000-yard season possible to him.

"I'm always looking up at 2,000, 2,500 yards," he said. "I'm making it simple for myself. ... If I go out and continue to play the way I'm playing, I think it will come."